feed: ensure line length of 80 characters
Ensure correct price role and value type, fixes #5

In the current version of the OpenMensa Feed specification (v2, see XSD here),
only the following price roles are valid: pupil, student, employee, other.

However, when building a feed, pyopenmensa does not enforce that restriction:
The result is that during development, the feed builders gracefully accept any
role name as input and you will only see your mistake when trying to add your
new canteen feed to openmensa.org which complains about XML validation errors.
I have already run into this problem.

I think that only the specified role names should be accepted in order to get
only feeds that are valid according to the specification.

-- cyroxx

This commit add value and type checks.